As is well known, the term "ferrite" is a generic term for groups of inorganic compounds containing Fe.sub.2 O.sub.3 as one of their components, most of them are ferromagnetic and are widely used in practical applications as magnetic materials. Fe.sub.2 O.sub.3 forms various compounds depending upon the properties of the other components of the material, and their combinations in turn form a huge number of composite ferrites and a wide variety of substitutional solid solutions and addition solid solutions.
A sintering step is employed in order to obtain a sintered body having the desired magnetic characteristics and mechanical strength by heating an article molded of powdered raw materials to a high temperature in an appropriate atmosphere, and increasing the density greatly while a solid phase chemical reaction is being carried out between the raw materials. This sintering temperature varies according to the composition of the raw materials, the shape of the powdered raw materials, the sintering method, etc. In general, the sintering temperature is between 1,200.degree. to 1,400.degree. C. for manganese and magnesium ferrites, 1,050.degree. to 1,200.degree. C. for nickel ferrites, 950.degree. to 1,100.degree. C. for copper ferrites, 1,050.degree. to 1,150.degree. C. for lithium ferrites, and 1,100.degree. to 1,250.degree. C. for barium ferrites and strontium ferrites. If sintering could be done at a temperature lower than these ranges, great practical advantages could be obtained not only because of the large savings of energy and improvements in productivity that can be accomplished, but also because the ferrites can be sintered simultaneously with other articles and materials (such as metal electrodes that decompose thermally at high temperatures), thereby making it possible to develop novel electronic components.
A pressure-sintering method or the like can be used for low temperature sintering. However, since this method requires special apparatus and has a low productivity, the shape and application of the products are very limited. As well as such a specific method, the following methods can be employed:
(1) A reduction of particle diameter. PA1 (2) Making the particle shape as spherical as possible. PA1 (3) The introduction of oxygen into the space lattice. PA1 (This is based upon the knowledge that the diffusion of oxygen ions is a rate-determining factor in the sintering of ferrites.) PA1 (4) The use of liquid phase sintering.
In the production of a ferrite, the raw materials go through the steps of mixing, calcining and milling and are thereby adjusted to be a powder of substantially spherical particles with a particle diameter of up to 1 .mu.m. If the particle size is reduced further, the moldability would drop impracticably. Accordingly, there is not much scope for change in methods (1) and (2) above. In method (3), a composition containing an amount of Fe.sub.2 O.sub.3 that is smaller than the stoichiometric amount is selected, and this has been used in the past. The above sintering temperature includes the effects of this method. The use of the liquid phase sintering of method (4) usually provides a low temperature sintering effect, but no technique has yet been found that provides a large effect for ferrites. The prior art techniques that provide such low temperature sintering effects deteriorate the magnetic characteristics markedly, especially the loss coefficient. In addition, the low melting-additives used to induce the liquid phase are usually expensive.